As a wafer for producing a device, such as a semiconductor integrated circuit, a silicon single crystal wafer grown by the CZ method is mainly used. If a portion near the surface of this silicon single crystal wafer is made defect-free as much as possible, a quality of the device will be improved. One of the most effective methods is an epitaxial wafer, and excellence thereof has been almost proved.
On the other hand, formation of a high-density defect (BMD) in a bulk of a wafer is more advantageous to the production of a device. Because there are quite a lot of opportunities where it is exposed to contamination with a heavy metal impurity during a device fabrication heat treatment, and the heavy metal may affect adversely on device operation, a demand for removing it from a portion near the surface which is a device fabrication range is often arisen. A gettering technique is a method of meeting the demand, and BMDs are sometimes formed in a bulk of a wafer as a gettering site in the gettering technique.
Although the silicon single crystal manufactured by Czochralski (CZ) method contains oxygen inevitably in a manufacturing process, it is possible to control the oxygen concentration and CZ-silicon wafers with various oxygen concentrations are manufactured according to the purpose. If these oxygen atoms are subjected to heat treatment, oxide precipitates will be formed in the inner part of the wafer. This is a main component of BMD. Not a little distortion of a crystal lattice exists around the BMDs, and a heavy metal impurity is captured by the distortion. This is a method called IG (Internal Gettering) among the various gettering techniques.
Generally, some methods can be considered to apply the IG method. The simplest method is a method of forming BMDs simultaneously during a device fabrication heat treatment. Although it is effective when the device fabrication heat treatment is performed at a high temperature, effectiveness cannot be achieved at the low temperature as 1000° C. or less. Especially the device process tends to be performed at lower temperature in recent years and it is impossible to expect formation of BMDs. In the case that a strong gettering capability is also necessary even when such a low-temperature process is used, there can also be adopted a method of forming BMDs before a device process. It is called DZ (Denuded Zone)-IG, which is a method wherein the oxygen atoms near a surface are out-diffused by a high temperature heat treatment and emitted out of the wafer, and then subjected to two-step heat treatment for formation of nuclei of oxygen precipitation and growth, to obtain a BMD density serving the purpose. However, according to the method, heat treatment is complicated, a long time is required and cost is very high.
As means for forming BMDs required for gettering in a bulk of a wafer easily, there have been recently used a method of doping nitrogen to a silicon single crystal to be pulled according to the CZ method. It is known that if nitrogen and oxygen exist together in silicon, a rate of formation of nuclei of oxygen precipitation will be raised, and thus BMDs can be formed easily. Since the silicon wafer manufactured by this method has a high BMD density, it has been considered that it is an optimal wafer for gettering.
The wafer which can realize simultaneously both of the gettering capability of the nitrogen doped wafer and an excellent surface quality is an epitaxial wafer wherein a nitrogen doped wafer is used as a substrate. It has been considered that it has a higher gettering capability compared with a conventional epitaxial wafer, since it contains BMD from the beginning. Actually, the wafer after subjected to a device fabrication heat treatment was characterized in having sufficient gettering capability, since BMD might be grown further.
However, there is a recent tendency of performing a device fabrication heat treatment by RTA (Rapid Thermal Annealing) in addition to a tendency of performing it at a low-temperature. That is, the device fabrication heat treatment which had taken several hours until now will be carried out in several seconds to several minutes. In the heat treatment for such a short time, it can be hardly expected that BMDs are formed or grown in the process. In the wafer after subjected to such a device fabrication heat treatment, a gettering capability is sometimes insufficient, even in the case that the BMD density detected in the bulk of the wafer is about 108 numbers/cm3 which is considered to be a sufficient BMD density.